1. Field of the Invention
The present invention relates to the field of wireless communication, and more particularly to transmit power level sense circuits for maintaining transmitter power at a desired level.
2. Prior Art
The preferred embodiment of the present invention is intended for use in the mobile receivers used in cellular and other communication systems, wherein the maximum transmitter power which may be used from the mobile unit back to a base station is limited by law or regulation. By way of example, a typical cellular telephone is limited in transmit power back to a base station to 200 milliwatts, whereas a cellular phone mounted in a vehicle and communicating through a vehicle-mounted antenna is limited to a transmit power of 500 milliwatts. While the present invention is not limited to use with such systems, the preferred embodiment is specifically intended for use in cellular telephones, and accordingly, unless indicated otherwise, the following description of the prior art, and for that matter of the preferred embodiment of the invention, is directed toward cellular telephone use of the invention, most specifically in the personal unit carried by an individual, sometimes referred to as a subscriber unit, herein generally referred to simply as a "cellular telephone."
The original cellular telephones were analog units representing primarily a miniaturization of radio voice communication systems. More recently, cellular telephones have been adapted for broader usage, including adaptations to serve as a wireless communication link for fax machines, computer modems, and the like. For this purpose, current cellular telephones are microprocessor controlled systems, wherein a user may manually execute the typical telephone functions of dialing, going off hook, going back on hook, etc., and of course be provided with a typical audio input and output functions of conventional land line telephone handsets. In addition, however, the microprocessor control allows operation of the various telephone functions by modems, fax machines, and the like, so that a modem or fax machine connected to the cellular telephone may operate through the cellular system much like the same equipment operates over land line telephone systems.
In order to get maximum performance from a cellular telephone, it is desired to maintain the transmit power of the telephone as close to the legal limit as possible, as this will reduce the occurrences of loss of connection due to weak signals, will improve voice transmission quality and reduce data error rates, resulting in increased data communication rates. In prior art cellular telephones, the transmit power is usually measured by measuring the amplitude of the transmit signal by way of a conventional rectifying diode-based amplitude detector on the transmit signal. Such power level sensing circuits have the advantage of simplicity to provide a signal which, in the older analog-controlled telephones, could be used to directly control the transmit power level, and in processor-controlled cellular telephones, may be readily digitized for processor control of the transmit power. However, the rectifying diode in the peak detector is temperature sensitive, so that a transmit power control system which will maintain the transmit power within the acceptable limit over the operating temperature range of the cellular telephone will actually limit the transmit power to a level which may be significantly below the allowed level at all except the extreme temperature of the operating temperature range. In that regard, cellular telephones may be called upon to operate over a wide temperature range because of the manner in which they are used. For instance, it is not at all uncommon for a cellular telephone to be kept in a briefcase in a car, or in the glove compartment of the car, subjecting the same to temperatures ranging from the temperature of a closed-up car sitting in the sun on a hot day, to temperatures of a cold winter morning of northern climates.